How to construct
a chimney crown
Make sure it allows for expansion
and contraction and sheds water
By Carolyn Schierhorn
W
hether made of precast
or cast-in-place concrete,
a properly constructed
chimney crown (also called a cap
or top plate) sheds water and
seals the top of the chimney. By
trapping air in the space between
the flue liner and the masonry,
the c r o w na l s oe n h a n c e s the chimney’s insulation (except in areas
governed by the Uniform Building Code, which requires this
space to be grouted).
But much of the time, chimney
crowns are poorly built, says Dale
Deraps of Advanced Chimney
Techniques in Jamestown, Mo. In
fact, poor crown construction is a
leading cause of chimney deterioration and failure.
Many crowns consist of a thin
mortar wash that erodes quickly,
shrinks, a n dc r a c k s , causing moisture penetration. A large area of
mortar, placed horizontally, will
not resist weathering and temperature changes as well as concrete. In addition, crowns commonly end flush with the edge of
the chimney, enabling water to
run straight down the masonry.
Crowns sometimes butt up
against the flue liner, failing to allow for its thermal expansion. Not
only can moisture enter through
this unsealed connection, but also
vertical expansion can lift the
crown, causing damage to the
masonry courses below.
What’s more, masons sometimes neglect to establish a bond
break between the crown and
This steel c ro w n form features a
built-in drip edge and 1⁄2x 1⁄2-inch
support tabs in each inside corner.
The tabs rest on the masonry, while
the form itself abuts the outside
edge of the masonry. Adjustable
c ro w n f o rm s can fit any size chimney—up to the length of the form
rails, which generally are 2 to 8
feet long; however, no more than
two-thirds of the length of the form
should extend beyond the chimney.
masonry. Differential expansion
will cause cracking and water
penetration. And some crowns
rest on combustible supports such
as plywood or tar paper, which
can burn and sag. Since plywood
absorbs water, it also can expand
and crack the crown.
Chimneys frequently suffer
water damage, freeze-thaw erosion, spalling,and efflorescenceas
a result of faulty crown construction, keeping chimney repair specialists busy. “For lack of a good
crown, I’ve taken large chimneys
straight to the ground and rebuilt them,” Deraps says. “These
are $10,000 to $12,000 jobs.”
Water penetration will shorten
the life of masonry or steel fireboxes, which often require extensive repairs in less than 10 years,
adds Jerry Isenhour of Concord,
N.C.-based The Chimney Doctor
Ltd., who is technical director of
the National Chimney Sweep
Guild (NCSG). Smoke chambers
also are frequently damaged by
water. “Water penetration will
erode parging and wash out mortar joints,” he says.
BIA guidelines
The Brick Institute of America
(BIA), which receives numerous
complaints about “disintegrating”
brick near the top of chimneys,
has issued the following guidelines for chimney crown construction (Ref. 1 and 2):
■ Never use mortar as the crown
material
■ Make sure precast concrete
crowns are at least 2 inches
Integral fiber reinforcement may be added to the chimney crown concrete
during mixing.
thick at the thinnest part
■ Reinforce cast-in-place concrete crowns
■ Make sure the crown extends
at least 21⁄2 inches beyond the
outside chimney wall; a drip
edge, or groove, should be incorporated on the underside of
this overhang—at least 1 inch
from the chimney wall
■ Slope the crown downward
from the flue liner to the edge
of the chimney wall
■ Don’t let the crown touch the
flue liner
■ Use a sealant between the flue
liner and the crown
■ Maintain the required distance
between the top of the installed
crown and the top of the flue
liner, as dictated by local building codes
■ M a k es u r e t h e r ei s a b o n db r e a k
between the crown and the masonry to allow for differential
expansion
Precast versus cast-in-place
This steel crown form features
integral clamps for fast and easy
adjustment.
Precast concrete crowns are
easy to install and provide consistent quality, but they generally
are used only in new construction
or on smaller jobs, such as 16x16inch chimneys.
Chimney repair work usually
is done by specialists who typical-
ly are small contractors, notes
NCSG executive director John
Bittner. The heavy weight of concrete (150 to 155 pounds per cubic foot) makes hoisting large
precast crowns impractical. It’s
much easier to carry 50 to 70
pounds at a time and build a 450pound crown in stages.
What’s more, adjustable steel
forms make it easier to construct
cast-in-place crowns today. Masons traditionally have built wood
forms by hand—a time-consuming process that must be repeated whenever the form wears out
or the mason tackles a chimney
of a different size. Steel forms
require only half the setup time
that wood forms do. Using Cclamps or built-in clamps, they
can be adjusted in seconds to fit
any size crown—up to the maximum length of the form.
Steel forms also last forever if
properly maintained. Oil them
before each use with a standard
form oil. And clean the concrete
residue off after each use.
Buying the right form
Steel form manufacturers generally offer form rails in 2- to 8foot lengths, which refers to the
maximum length of crown the
form will create. Don’t use a large
masonry isn’t level, the form will
wobble and the crown will have
imperfections.
Also, look for a steel form with
a welded-in drip edge to save setup time, Deraps suggests. It produces an accurate and consistent
drip edge.
A chimney crown needs a drip
edge so that water drips away
from the chimney, not down its
surface. Water can run horizontally, but it can’t run horizontally
and then back up again.
Creating the drip edge
To construct the crown support
grid, cut the ends of angle irons in
half so they can lay flat on the
masonry.
form on a 16x16-inch chimney,
where 4 or 5 feet might be sticking out and the form will be out
of balance. Follow this rule of
thumb when buying a form: No
more thantwo-thirds of the length
of the form should extend beyond
the chimney.
Another consideration when
buying a form is how easily it can
be attached to the chimney and
stripped out after the concrete
has set. One manufacturer produces a form with a small 1⁄2x1⁄2inch tab in each corner that rests
on top of the masonry, while the
form itself abuts the outside edge
of the masonry (see photo, page
000). This type of form is simple
to install and can be stripped out
easily because only the small
tabs are covered by concrete.
A second type of steel form requires the contractor to strap angle irons or brackets to the chimney before positioning the form
rails. This type may be more timeconsuming to install.
A third type of form rests right
on the masonry, allowing about 1⁄4
inch around the chimney’s perimeter to become embedded in concrete. This type may be harder to
strip out unless a good release
agent is used. Additionally, if the
Install the steel crown form
according to manufacturer’s directions. If the form doesn’t have
a built-in drip edge, you can create one by placing a bead of
caulk on the form. Alternatively,
some contractors create the
groove by placing a cotton rope
in the form, which gets stripped
out later. The drip edge should
be located at least 1 inch from
the chimney wall o nt h e underside of the crown.
Photo shows components of chimney crown in place: Flat metal
plates cover t h eg r i da n dm a s o n ry,
s e rv i n ga s a b o n db re a ka n da ss u pport f o rt h ec o n c re t e ; flues are
wrapped in metal sleeves to allow
for silicone expansion joints; and
wire mesh reinforcement in the concrete inhibits cracking.
Support system
and bond break
Next, install the crown support
system, which covers the open area
between the flue or flues and the
masonry and serves as a bridge
from masonry to masonry. If contractors were to pour concrete
without this support, the whole
chimney would become filled. The
support system also establishes a
bond break between the concrete
and masonry. It’s important to
have a bond break because masonry and concrete expand and
contract at different rates.
To construct the support system, use angle irons of the appropriate length, cutting the ends in
half so they can lay flat on the masonry (see photo, above left). With
these angle supports, build a grid
that goes around the flue liner and
across the span of the chimney.
Then lay flat metal plates on top
of this grid. (Reinforced cement
backing board may also be used,
but don’t use combustible materials such as plywood or tar paper.)
Place tape around the perimeter
to seal the plates to the form.
Also, wrap the flue liner with a
compressible material to allow a
1
⁄4- to 3⁄8-inch separation between
the concrete and flue liner. “The
tile needs to be able to move up
and down slightly, as well as expand outward, so it doesn’t
crack the crown,” Bittner says.
Like the masonry and concrete,
the flue liner and concrete have
different coefficients of expansion.
Onemanufacturer recommends
placing metal sleeves around the
flue liner, which remain in place
and allow room for the silicone
expansion joint. But Isenhour
p r e f e r st o use 1⁄4- i n c h - t h i c kc er amic wool. “It can be either stripped
out and the space siliconed over,
or left in and siliconed over; it has
the necessary flexibility,” he says.
Put in an expansion joint that
will be flexible and hold the bond
break 1⁄4 to 3⁄8 inch away from the
flue. This expansion joint should
be made of silicone sealant,
A chimney crown should slope downward from the flue and extend beyond
the chimney wall.
which seals out water but allows
some movement, and may be
added when the concrete is cast.
Don’t wrap tar paper around
the flue liner to act as an expansion joint, as some masons do.
While allowing for expansion up
and down, it doesn’t let the flue
liner expand outward enough and
will crack the crown.
Whether cast-in-place or precast, the crown should be seamless and finished so water will
not collect on the top surface. The
crown should be at least 4 inches
t h i c kt o avoidpremature cracking.
Make sure the flue liner extends at least 2 inches beyond
the top of the chimney, but check
local building codes for the requirements in your area. This
extension permits attachment of
a rain hood and keeps ice and
snow that accumulates on the
crown from dropping into the
flue. To avoid excessive cooling
and condensation, don’t project
the flue liner more than 6 inches
above the crown.
For a cast-in-place concrete
crown, one manufacturer recommends a rich concrete mix of 1
part portland cement, 2 parts
concrete sand, and 2 parts coarse
aggregate. It may also contain an
integral water repellent.
Isenhour uses a batch mix
that, per cubic foot, consists of
21 pounds portland cement, 42
pounds dry masonry sand, and
63 pounds of aggregate no larger
than 1⁄4 to 1⁄2 inch. To this mixture,
he adds 3 tablespoons of an airentraining admixture.
Add reinforcement
A properlyconstructed chimney
crown should not crack; but if it
does, reinforcement can prevent
the crack from expanding. If a
hairline crack does develop, reinforcement can make the crack
manageable; the crack can be
g r o u n do u ta n d c a u l k e dt op r e v e n t
it from separating any further.
Integral fiber reinforcement
may be added to the concrete
during the mixing stage. One
supplier advocates this type of reinforcement because it is threedimensional, reinforcing up and
down, as well as from side to side.
However, the most common
m e t h o do fr e i n f o r c e m e n t u s e sw i r e
mesh, which should be placed
mid-depth in the filled form. Be
sure to select the right gauge and
size. I ft h es c r e e n of t h ew i r em e s h
is too small, bonding will not occur from one side of the mesh to
the other. For chimneys, use at
least a 1x1-inch mesh for optimum bonding through the holes.
When casting a crown in place,
take the time to do the work accurately. Altering the crown after
it is cast will make it look sloppy
and fit poorly.
Fill the form with concrete,
making sure that the wire mesh
reinforcement, if used, is positioned properly. Don’t let t h em e s h
extend to the edge of the form or
the outside surface. To prevent
rust, there should be a 1-inch
clearance between mesh and form
and mesh and flue tile.
Cast the concrete so it slopes
downward from the flue on all
sides, shaping it with a wooden
float. “You have to stay with it
awhile and play with it,” Isenhour points out. “You can’t just
dump it there and leave it.”
Then Isenhour uses the rough
side of a brick to smooth out rough
spots and small surface cracks.
Finish the crown using magnesium and sponge floats to smooth
the top and sides and a broom to
make the texture more uniform.
Tap the sides of the form gently
with a hammer to release trapped
air bubbles. A vibrator connected
to the form also will force out the
air bubbles.
T h e nc o v e r t h ec r o w nw i t h plastic and allow the concrete to set
overnight. Since weather conditions affect the curing rate of concrete, check the concrete for hardness before removing the forms.
To strip the form, follow manufacturer’s directions. Be sure to
scrape the form rails clean and
oil them lightly for storage.
Use a tool called an edger to
chamfer the edges of the concrete crown to the desired angle.
Besides enhancing the crown’s
appearance, this increases its
strength. Left sharp, the edges
of a cured concrete crown may
chip.
References
1. “Residential Chimneys Design and Construction,” BIA Technical Notes on Brick
Construction, Number 19B, Brick Institute
of America, 11490 Commerce Park Dr.,
Reston, VA 22091.
2. “Proper Chimney Crowns,” Engineering
and Research Digest, BIA.
PUBLICATION #M930413
Copyright © 1993, The Aberdeen Group
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